Optical communication system, and transmitter and receiver apparatus therefor

ABSTRACT

An optical communication system having a transmitter in which a pair of optical signals having different frequencies are modulated using a duobinary encoding scheme, and then multiplexed using polarization division multiplexing. Advantageously, the frequency difference between the two signals can be less than the data rate conveyed by each signal, resulting in a narrow spectral bandwidth, while still allowing demultiplexing at a receiver using simple bandpass filters and without the need of any form of polarization tracking. A receiver has a beam splitter for splitting the received optical signal into two portions which are each directed, via respective bandpass filters centred at slightly different frequencies, to respective detectors. Advantageously, the frequency difference between the frequencies at which the bandpass filters are centred can be less than the data rate of a detected signal. The receiver does not require any polarization tracking or balancing, and accordingly is straightforward to implement

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical communication system inwhich an optical beam is modulated in accordance with data in atransmitter, and the modulated optical beam is then transmitted to aremote receiver which recovers the data. The invention has particular,but not exclusive, application to a so-called 40G optical communicationnetwork at which data is communicated along a data pipe at a rate of 40Gigabits per second (Gbps) or more.

2. The Background Art

In recent years, the need to increase data rates in opticalcommunication to the benchmark figures of 40 Gbps and 100 Gbps hasprompted much research. One problem with increasing data rates is theconsequent increase in frequency bandwidth, which is problematic due toincreased dispersion in optical fibers and also because an increase infrequency bandwidth requires a greater frequency spacing of datachannels in a wavelength division multiplexing (WDM) system.

The use of optical duobinary modulation, in which a data signal is addedto a one-bit delayed version of itself to generate a three level signal,has attracted attention due to its narrow bandwidth in comparison with abinary non-return-to-zero (NRZ) modulated signal. In practice, opticalduobinary modulation typically employs a precoder to performdifferential encoding in order to prevent error propagation. In order tomaintain the bandwidth advantage when using such a precoder, one binarylogic level output by the precoder is converted to a low amplitude stateof the optical signal while the other binary logic level output by theprecoder is converted to high amplitude states of the optical signalhaving opposite phases. At the receiver, conveniently the low amplitudestate is converted to one binary logic value while both the highamplitude states are converted to the other binary logic value torecover the original data signal.

Other modulation techniques deployed include phase-shift keying (DPSK)and quadrature phase shift keying (QPSK), particularly in differentialformat. In addition, polarization division multiplexing has been used tofurther increase the data rates by employing two optical signals at thesame frequency but with orthogonal polarizations. Polarization divisionmultiplexing typically requires, however, a complex receiver due to thedifficulty in separating the two optical signals at the receiver withacceptable levels of crosstalk.

SUMMARY OF THE INVENTION

One aspect of the present invention provides for a transmitter in whicha pair of optical signals having different frequencies are modulatedusing a duobinary encoding scheme, and then multiplexed usingpolarization division multiplexing. Advantageously, the frequencydifference between the two signals can be less than the data rateconveyed by each signal, resulting in a narrow spectral bandwidth, whilestill allowing demultiplexing at a receiver using simple bandpassfilters and without the need of any form of polarization tracking.

Another aspect of the invention provides for a receiver having awavelength-dependent beam splitter arrangement for splitting a receivedoptical signal into two portions which are each directed to respectivedetectors. A first spectral component at a first frequency ispreferentially split into the first portion, and a second spectralcomponent at a second frequency is preferentially split into the secondportion. Advantageously, the frequency difference between the first andsecond frequencies can be less than the data rate of a detected signal.The receiver does not require any polarization tracking or balancing,and accordingly is straightforward to implement.

A further aspect of the invention provides a Dense Wavelength DivisionMultiplexing (DWDM) optical communication system in which a plurality oftransmitters generate a modulated optical signal by using polarizationdivision modulation to combine two optical signals at slightly differentfrequencies, modulated in accordance with a duobinary encoding scheme,to generate respective optical data signals. The optical data signalsare combined using wavelength division multiplexing, and transmittedover an optical fibre to a demultiplexer which demultiplexes the opticaldata signals. Each optical data signal is then split into two portions,and each portion is directed via a respective bandpass filter to arespective detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the main components of an opticalcommunication system forming a preferred embodiment of the invention;

FIG. 2 is a graph showing the variation of electric field for an opticalsignal output by a Mach-Zehnder modulator with variation of an appliedelectrical potential;

FIG. 3 is a graph showing an exemplary waveform input into a low passfilter forming part of a transmitter of the optical communication systemof FIG. 1;

FIG. 4 is a graph showing the waveform output by the low-pass filter inresponse to the exemplary input waveform illustrated in FIG. 3;

FIG. 5 is a graph showing an exemplary frequency spectrum of the outputof a transmitter of the optical communication system illustrated in FIG.1;

FIG. 6 is a graph showing transmissivity against frequency for abandpass filter in a receiver forming part of the optical communicationsystem illustrated in FIG. 1;

FIG. 7 is a block diagram showing the main components of a firstalternative receiver for the optical communication system illustrated inFIG. 1;

FIG. 8 is a block diagram showing the main components of a secondalternative receiver for the optical communication system illustrated inFIG. 1; and

FIG. 9 is a block diagram of a DWDM optical communication systemincluding optical communication in accordance with the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of the present invention will now be described, includingexemplary aspects and embodiments thereof. Referring to the drawings andthe following description, like reference numbers are used to identifylike or functionally similar elements, and are intended to illustratemajor features of exemplary embodiments in a highly simplifieddiagrammatic manner. Moreover, the drawings are not intended to depictevery feature of actual embodiments nor the relative dimensions of thedepicted elements, and are not drawn to scale.

As shown in FIG. 1, in an optical communication system according to thepresent invention a transmitter 1 transmits a modulated optical signalthrough an optical fiber 3 to a receiver 5. The optical signal ismodulated in accordance with first and second data signals which areinput to respective precoders 7 a, 7 b of the transmitter 1. In thisembodiment, each data signal conveys data serially at a data rate of 22Gigabits per second (Gbps). The pair of data signals may be formed froma single data signal at 44 Gbps.

Each of the precoders 7 performs differential encoding. In particular,in each precoder the input data signal is inverted and then input intoone input of an exclusive-OR gate, and the output of the exclusive-ORgate for each clock cycle is input into the other input of theexclusive-OR gate for the following clock cycle. The output of theexclusive-OR gate also forms the output of the precoder 7.

The output of each precoder 7 a, 7 b is input to a respective 2V_(π)drive circuit 9 a, 9 b, with each 2V_(π) drive circuit 9 applyingcontrol voltages to a corresponding Mach-Zehnder modulator 13. As thoseskilled in the art will appreciate, a Mach-Zehnder modulator splits areceived coherent optical signal into two light beams which are directedthrough respective arms of the Mach-Zehnder modulator and thenrecombined. A variable optical path difference is introduced into one orboth of the light paths in order to vary the amplitude of the recombinedoptical signal.

In this embodiment, each 2V_(π), drive circuit 9 has a pair of V_(π)drive circuits, with the output of each V_(π) drive circuit being input,via a respective low-pass filter 11, to an electrode associated with arespective arm of corresponding Mach-Zehnder modulator (MZM) 13. One ofthe V_(π) drive circuits is driven by the output of the correspondingprecoder 7 while the other of the V_(π) drive circuits is driven by theinverse of the output of the corresponding precoder 7 so thatdifferential driving is performed. Each Mach-Zehnder modulator 13 isbiased at a level where the optical path difference between the twopaths is 180°, resulting in a null output as the light travelling downone path destructively interferes with the light travelling down theother path. The 2V_(π) drive circuits 9 are configured such that apotential difference of amplitude V is applied across the electrodesassociated with the arms of the MZM 13, with the polarity of the appliedvoltage dependent on the binary logic level output by the correspondingprecoder 7. The application of the potential difference V with onepolarity results in a maximum amplitude of the recombined optical signaloutput by the MZM 13 with a first phase while the application of thepotential difference V with the other polarity results in a maximumamplitude of the recombined optical signal output by the MZM 13 at asecond phase which is 180° out of phase with the first phase. In otherwords, as illustrated in FIG. 2, the electric field strengths E of therecombined optical signal output by the MZM 13 when the potentialdifference V is applied with opposite polarities are of equal amplitudebut opposite sign.

The low-pass filters 11 are configured such that the output of eachlow-pass filter 11 substantially corresponds to the average of thevoltage levels output by the corresponding 2V_(π) drive circuit 9 forthe last two data bits. Accordingly, if the output of a V_(π) drivecircuit 9 corresponds to a sequence of two different bits, then thevoltage output by the low-pass filter is effectively zero, whereas ifthe two bits are the same then the voltage output by the low pass filtercorresponds to the input voltage. This is a conventional way ofimplementing a duobinary encoding scheme.

In this embodiment, the low-pass filters 11 are 5^(th) order Besselfilters which provide a substantially flat group delay up to 13.4 GHz.FIGS. 3 and 4 respectively show an exemplary input to a low-pass filter11 and the corresponding output of the low-pass filter 11.

First and second lasers 15 a, 15 b output coherent light beams which areinput to respective ones of the modulators 13 a, 13 b. In thisembodiment, the first laser 15 a outputs a coherent optical beam at afirst wavelength λ₁ and the second laser 15 b outputs a coherent lightbeam at a second wavelength λ₂, with the frequency difference betweenthe two laser equal to 16 GHz. This frequency difference is thereforeless than the data rate of one of the data signals. Further, the outputsof the first and second lasers 15 a, 15 b have linear polarizationswhich are mutually orthogonal to each other. A polarization beamcombiner 17 combines the two outputs of the MZMs to form the outputsignal of the transmitter 1, and this output signal is coupled into theoptical fibre 3. The different polarization states of the outputs of theMZMs reduces interference between the data of the first and second datasignals. FIG. 5 shows the frequency spectrum of an exemplary output ofthe transmitter 1. It will be seen that there are two local maxima,which correspond to the wavelengths of the first and second lasers 9.

Table 1 illustrates states of the transmitter 1 for an exemplary datastring.

TABLE 1 States of components of the transmitter 1 for an exemplary datastring. Clock Cycle −1 0 1 2 3 4 5 Data 0 1 0 1 1 1 Precoder Output 0 11 0 0 0 0 Drive Circuit Output −V V V −V −V −V −V Low-pass Filter Output0 V 0 −V −V −V MZM Output 0 E 0 −E −E −E

In table 1 it will be seem that the output of the MZM 13 corresponds toa duobinary encoded version of the data signal in which the binary logicstate “1” is represented by an electric field amplitude E at two phaseswhich are 180° out of phase with each other. Accordingly, a spectralcomponent at wavelength λ₁ is modulated in accordance with the firstdata signal and a spectral component at wavelength λ₂ is modulated inaccordance with the second data signal. At the receiver, a data signalcan be recovered simply by detecting the amplitude of the electric fieldstrength at the corresponding wavelength.

Returning to FIG. 1, after passing through the optical fiber 3, thesignal output by the transmitter 1 is input to the receiver 5 where itis split into two equal portions by a beam splitter 19. In thisembodiment, the beam splitter 19 is wavelength insensitive so that thespectral distributions of each of the split portions are the same. Onesplit portion is input to a first bandpass filter 21 a and the othersplit portion is input to a second bandpass filter 21 b. The firstbandpass filter 21 a is centred at λ₁ while the second bandpass filter21 b is centred at λ₂. The first and second bandpass filters 21 a,21 bboth have a 3 dB bandwidth of 16 GHz, so that light transmitted by thefirst bandpass filter 21 a generally originates from the first laser 15a and light transmitted by the second bandpass filter 21 b generallyoriginates from the second laser 15 b. FIG. 6 illustrated how thetransmissivity of a bandpass filter 21 varies with frequency.

The light transmitted by the first bandpass filter 21 a is detected by afirst detector 23 a to recover the first data signal and the lighttransmitted by the second bandpass filter 21 b is detected by a seconddetector 23 b to recover the second data signal.

It will be appreciated that the light output from each bandpass filter21 could be amplified using an optical amplifier prior to detection.

In an embodiment, the components of the transmitter 1 are formed in anintegrated optical circuit, and similarly the components of the detector5 are formed in an integrated optical circuit.

In the receiver 5 discussed above, the beam splitter 19 and the firstand second bandpass filters 21 a,21 b form a wavelength-dependent beamsplitting arrangement. Other forms of wavelength-dependent beamsplitting arrangements are possible. For example, as shown in FIG. 7, inan alternative embodiment a receiver 5′ has a wavelength-dependent beamsplitter arrangement in the form of an optical de-interleaves 27 whichdirects a first optical signal predominantly comprising a first spectralcomponent to a first detector 23 a and a second optical signalpredominantly comprising a second spectral component to a seconddetector 23 b. More generally, as shown in FIG. 8, in an embodiment areceiver 5″ has a wavelength-dependent beam splitter arrangement in theform of a wavelength demultiplexer 29.

Due to the narrow bandwidths of the transmitted optical signals,transmitters and receivers according to the present invention are wellsuited to a DWDM optical communication system. In a DWDM, multiplechannels at different wavelength are multiplexed into a single fibercommunications window, usually the window around 1550 nm to takeadvantage of the devices available at that wavelength. As shown in FIG.9, a plurality of transmitters as described above each output an opticalsignal having two components centred at slightly different frequencies,with the frequencies used in one transmitter 1 being spaced from thefrequencies used in all the other transmitters 1. The output signals areinput to a wavelength multiplexer 31 which combines the output signals,and the combined output signal is transmitted through the optical fiber3. Following transmission through the optical fiber 3, the transmittedsignal is demultiplexed by the wavelength demultiplexer 33 to recoverthe optical signals having two components at slightly differentfrequencies, and these optical signals are input into respectivereceivers 5 as described above.

In the embodiment illustrated in FIG. 1, two lasers 9 a, 9 b are usedhaving orthogonal linear polarizations. It will be appreciated thatdifferences in the polarization state could be used, for exampleorthogonal circular polarizations. Alternatively, two lasers emittinglight beams having identical polarizations could be used, with thepolarization state of one light beam being altered prior to combiningwith the other light beam in the polarization beam combiner. It will befurther appreciated that a single laser can be used to generate twolight beams at slightly different frequencies.

1. A transmitter comprising: a first light source for generating a firstcoherent light beam at a first frequency; a first encoder for encoding afirst data signal using a duobinary encoding scheme to generate a firstencoded signal; a first modulator for modulating the first coherentlight beam in accordance with the first encoded signal to generate afirst modulated light beam, wherein the first modulated light beam has afirst polarization state; a second light source for generating a secondcoherent light beam at a second frequency different from the firstfrequency; a second encoder for encoding a second data signal using aduobinary encoding scheme to generate a second encoded signal; a secondmodulator for modulating the second coherent light beam in accordancewith the first second encoded signal to generate a second modulatedlight beam, wherein the second modulated light beam has a secondpolarization state that is different from the first polarization state;and a combiner arranged to combine the first modulated light beam andthe second modulated light beam for form a combined light beam foroutput to a communication channel.
 2. A transmitter according to claim1, wherein the first and second data signals have a data rate which isgreater then a frequency difference between the first frequency and thesecond frequency.
 3. A transmitter according to claim 1, wherein thefirst encoder and the second encoder comprises: a precoder fordifferentially encoding the first data signal to generate a differentialsignal; a drive circuit for generating a drive signal in dependence onthe differential signal; and a low pass filter for filtering the drivesignal to generate a duobinary drive signal.
 4. A transmitter accordingto claim 3, wherein the precoder comprises: an inverter for invertingthe first data signal to generate an inverted data signal; and a logicgate for performing an exclusive-OR logic function on the inverted datasignal and a feedback data signal to generate an output signal, thefeedback data signal corresponding to a time-delayed version of theoutput signal.
 5. A transmitter according to claim 3, wherein the lowpass filter is a fifth order Bessel filter.
 6. A transmitter accordingto claim 1, wherein one or both of the first modulator and the secondmodulator comprises a Mach Zehnder modulator.
 7. A transmitter accordingto claim 1, wherein the first polarization state and the secondpolarization state are linear polarization states which are orthogonalto each other.
 8. A transmitter according to claim 1, wherein thecombiner is a polarization beam combiner.
 9. A transmitter according toclaim 1, wherein the first frequency and the second frequency are 20 GHzor more.
 10. A transmitter according to claim 9, wherein a frequencydifference between the first frequency and the second frequency is lessthan 20 GHz.
 11. A receiver comprising: a wavelength-dependent beamsplitter arrangement operable to split a received optical signal, whichis modulated in accordance with a data signal and has a first spectralcomponent centred at a first frequency and a second spectral componentcentred at a second frequency different from the first frequency, into afirst optical signal and a second optical signal, wherein the firstoptical signal comprises more of the first spectral component than thesecond spectral component and the second optical signal comprises moreof the second spectral component than the first spectral component; afirst detector for detecting the first optical signal; and a seconddetector for detecting the second optical signal, wherein the firstdetector and the second detector are operable to detect a data signalhaving a data rate that is greater than the frequency difference betweenthe first frequency and the second frequency.
 12. A receiver accordingto claim 11, wherein the wavelength-dependent beam splitter arrangementcomprises: a beam splitter operable to split the received optical signalinto a first split optical signal and a second split optical signal; afirst bandpass filter operable to filter the first split optical signalto form the first optical signal, the first bandpass filter beingcentred at the first frequency; and a second bandpass filter operable tofilter the second split optical signal to form the second opticalsignal, the second bandpass filter being centred at the secondfrequency.
 13. A receiver according to claim 12, wherein the firstbandpass filter and the second bandpass filter have a 3 dB bandwidthcorresponding to the frequency difference between the first frequencyand the second frequency.
 14. A receiver according to claim 11, whereinthe wavelength-dependent beam splitter arrangement comprises an opticalde-interleaver.
 15. A receiver according to claim 11, wherein thewavelength-dependent beam splitter arrangement comprises a wavelengthdemultiplexer.
 16. A receiver according to claim 11, wherein the firstdetector and the second detector are operable to detect a data signalhaving a data rate of 20 GHz or more.
 17. A receiver according to claim11, wherein the frequency difference between the first frequency and thesecond frequency is less than 20 GHz.
 18. An optical communicationsystem comprising: a plurality of transmitters for generating modulatedoptical signals, each modulated optical signal being centred at arespective different optical frequency; a multiplexer for multiplexingsaid modulated optical signals for simultaneous transmission over anoptical communication link; a demultiplexer for demultiplexing saidmodulated optical signals following transmission over the opticalcommunication link; and a plurality of receivers for detecting themodulated optical signals, wherein each transmitter is arranged togenerate a pair of duobinary modulated optical signals centred atrespective optical frequencies separated by less than the data rate ofeach of the pair of duobinary modulated optical signals.